1. Field of the Invention
The present invention relates to a method of synthesizing a variable frequency signal, and a synthesizer for a variable frequency signal using the method.
2. Discussion of Related Art
In general methods of synthesizing variable frequency signals and general variable frequency signal synthesizers, there are a one-point modulation scheme in which a modulated signal is supplied to a delta sigma modulator included in a phase locked loop (PLL) to change a frequency division ratio, and a two-point modulation scheme in which a modulated signal is supplied to a voltage controlled crystal oscillator (VCXO) or a voltage controlled oscillator (VCO) as well as the delta sigma modulator included in a phase locked loop (PLL).
A variable frequency signal synthesizer uses a phase locked loop, and the phase locked loop has a low pass characteristic itself. Accordingly, in the one-point modulation scheme in which a frequency division ratio is changed in the phase locked loop to obtain a variable frequency signal, the modulation bandwidth of the frequency signal synthesized through the scheme is restricted within the bandwidth of the phase locked loop. The two-point modulation scheme is designed to achieve a wider modulation bandwidth than the one-point modulation scheme, by supplying modulated signals to the VCXO or VCO complementary to the phase locked loop so as to have a high pass characteristic. However, in the two-point modulation scheme, a mismatch between the two modulation gains may occur, and a timing mismatch may occur. In the conventional two-point modulation scheme, a calibration circuit may be necessary to compensate the gain mismatch and the timing mismatch. In addition, even when the calibration circuit is included, it is difficult to fully compensate the gain mismatch and the timing mismatch. In addition, due to the calibration circuit, a variable frequency signal synthesizer may have a complicated structure and a large die size, and power consumption thereof may be large.
Furthermore, in variable frequency signals synthesized using a conventional method, the feasible modulation depth is just several thousand parts per million (ppm) of the center frequency, or the modulation period is relatively long, for example, several milliseconds.